The invention concerns a process for the manufacture of dense amorphous quartz glass granulate by producing a porous granulate from SiO2 powder and vitrifying the granulate.
Amorphous SiO2 powder is obtainable for example by flame hydrolysis or oxidation of silicic compounds, by hydrolysis of organic silica compounds by the so-called sol-gel process or by hydrolysis of inorganic silica compounds in a liquid. For example, amorphous SiO2 powder having a high specific surface area ranging from 40 m2/g to about 400 m2/g is obtained in large quantities as a byproduct during the production of synthetic quartz glass for optical wave guides. However, re-use of the powder by melting is problematic. Due to their low apparent density the powders cannot be melted directly into transparent low-bubble quartz glass bodies. Wet granulation processes are for example commonly used to increase the density of the powder, whereby an agglomeration in form of porous granulate is produced from aqueous colloidal dispersion of such SiO2 powders by constant mixing or agitation while moisture is gradually being removed.
In a first process of this kind according to DE A1 44 24 044 it is proposed to treat an aqueous suspension of pyrogenuously produced silicic acid powder in a mixing container with rotating agitators whose rotational velocity during a first mixing phase is set at between 15 and 30 m/s and in a second mixing phase at 30 m/s or more. A coarse granulate mass is obtained after the first mixing phase. The degree of density of the said mass is increased by addition of silicic acid powder and in a second mixing phase the coarse mass is reduced by intensive mixing and beating. Water emerges on the surface of the granular mass, and gluing of the granulate is prevented by addition of more silicic acid powder. The porous and pourable SiO2 obtained in this fashion is then dried and sintered at 1000xc2x0 C. to 1200xc2x0 C. for solidification.
U.S. Pat. No. 5,604,163 describes a process for the manufacture of powder from synthetic quartz glass of the kind described initially. A gel produced according to the sol-gel method from tetramethoxysilane and water is rapidly dried in vacuum whereupon it breaks up while forming SiO2 granulate. The granulate having a particle size ranging between 100 xcexcm and 500 xcexcm is then placed in a sintering container of quartz glass and heated up in batches in an electric furnace at a rate of 200xc2x0 C./hr to a temperature of 1150xc2x0 C. and kept at that temperature for 35 hours. The quartz glass granulate obtained in this manner can be then used for quartz glass products by conventional methods such as the Verneuil method.
A pore-free quartz glass granulate is preferable in order to avoid bubble formation during melting. However, the quartz glass granulate obtained according to the known process may contain gas residues which result in bubble formation. Reduction of residual gases by longer sintering or higher sintering temperatures leads to increased time requirements and higher cost. In addition, higher sintering temperatures also encounter limits because granulate particles soften at higher temperatures and agglomerate into an undefined porous quartz glass mass.
Especially at high temperatures the quartz glass granulate may be contaminated by the material of the sintering container. Even though the risk of contamination can be reduced by the use of suitable containers, made for example of highly pure quartz glass, such containers are costly. In addition, sintering containers of quartz glass are not suitable for temperatures above about 1400xc2x0 C.
The object of the invention is therefore to provide an economical process for the manufacture of dense, highly pure quartz glass granulates.
The object is achieved on the basis of the process described initially in that the porous granulate is finely dispersed in a fuel gas flame and is vitrified in the fuel gas flame.
The term xe2x80x98granulatexe2x80x99 is understood to mean opaque pore-covered SiO2 granules which are composed of a plurality of primary particles; by contrast, vitrified granulates are transparent pore-free SiO2 granules having an amorphous structure.
In the process according to the invention the porous SiO2 granulate is exposed to a fuel gas flame and is finely dispersed, heated and vitrified therein. The fuel gas flame is more flexible as concerns temperature than in the known process, and especially the fuel gas flame permits higher temperatures. The granulate can be exposed to very high sintering temperatures in the fuel gas flame without sintering into agglomerates. At the same time any problems linked to sintering containers such as furnaces or melting pots, are avoided. In addition, contamination of the SiO2 granulate by contact with the walls of sintering or vitrification containers is avoided.
During the passage through the fuel gas flame the pores of the granulate collapse, resulting in an amorphous and dense quartz glass granulate. High temperatures accelerate the out-diffusion of gas remnants from the porous granulate and facilitate achievement of as high a density of the quartz glass granulate as possible, reducing the required sintering time from hours to seconds.
The fuel gas flame is generated by combustion of hydrogen containing components such as hydrogen itself, or carbon hydrogen compounds such as propane or acetylene. Reaction partners may be oxygen, oxygen compounds, halogens and halogen compounds.
It is of substance that the granulate particles do not agglomerate during the vitrification. Agglomeration is prevented in that the granulate is dispersed in the flame in fine distribution and is exposed to the flame in such fine distribution. For example, the granulate may be blown into the flame, sprayed or poured in. The individual granulate particles are heated in the fuel gas flame to high temperatures within a short time period while separate from one another due to the fine dispersion so that they cannot become glued together. The fine distribution assures that all the particles are exposed to the flame evenly and, furthermore, at a particularly high temperature, and are compacted.
The process according to the invention allows a continuous manufacture of SiO2 granules in that the granulate is being continuously fed into the fuel gas flame.
Particularly simple is a procedure whereby the granulate is poured into the fuel gas flame. Here the granulate is poured from above in a finely distributed form into the fuel gas flame. The direction of the flame is not of substance; it may be pointed vertical to the direction of the falling material, parallel to it or on a diagonal.
In an equally preferred procedure the granulate is supplied to the fuel gas flame in a gas stream. The gas stream can simultaneously assist in the local distribution of the granulate in the fuel gas flame by being used for whirling of the poured granulate so that individual granulate articles are carried along by the gas stream in the direction of the fuel gas flame. The gas stream may for example generate a whirling bed of the poured material and the material can be treated chemically and thermally at the same time. The gas stream can also serve as carrier for supplying the granulate to a burner, the latter also producing the fuel gas flame, in that the gas stream is charged with the granulate and fed into the burner.
In an alternative method thereto it has also been shown to be advantageous for the granulate to be fed into the fuel gas flame by means of a vacuum. In this method the granulate is suctioned into the fuel gas flame. The vacuum may be for example generated within the fuel gas burners by equipping it with a venturi jet into which the granulate is fed.
It has been shown to be advantageous to adjust the fuel gas flame to a temperature of at least 1600xc2x0 C., but preferably in the range from 2000xc2x0 C. to 2500xc2x0 C. However, a precise measurement of the fuel gas flame temperature is difficult for, among others, the introduction of the SiO2 granulate into the fuel gas flame lowers the flame temperature in dependence on the rate of granulate throughput. The fuel gas temperature is understood as the maximum temperature within the fuel gas flame without the addition of the granulate. The fuel gas flame reaches temperatures where the individual granulate particles melt completely which, depending on the type and granule characteristics of the granulate and the processing conditions can lead to a desired spherification of the SiO2 granulate. However, there is danger of agglomerate formation if the softened particles are allowed to come in contact with one another or with the walls before at least the surface of the individual particles has again cooled and solidified. Of substance in this case is not just the flame temperature but also the volume of the individual particles, the duration of a particle""s stay in the fuel gas flame, the cooling rate and the duration of the cooling phase before contact with a wall. However, these parameters can be optimized by an expert in the field on the basis of a few experiments. Formation of agglomerates can be also avoided by reducing the fine particle content of the granulate.
The fuel gas flame is advantageously provided with a halogen containing component. The halogen containing component serves for example to purify the granulate by reacting with contaminants in the granulate and forming volatile halogenides; to reduce the OH content of the vitrified granulate or for the adjustment of other chemical or physical characteristics of the quartz glass such as for example viscosity. Halogen containing components may be for example fluoride, chlorine, bromide, iodide, gaseous chemical compounds of these elements and mixtures of these elements and compounds. The halogen containing component may be a component of the fuel gas for the generation of the fuel gas flame. The fuel gas flame is formed in this case through an exothermic reaction of the hydrogen containing component and the halogen containing component. In this method of proceeding the SiO2 granulate is at the same time produced and purified by the fuel gas flame.
It has been shown to be particularly advantageous to precompact the granulate in a sintering step before the vitrification. The precompacting may be accomplished by sintering of the granulate in the fuel gas flame, by electrical heating or by microwave energy. For example, the granulate is supplied to the fuel gas flame multiple times and in the course of the sintering step none or only partial vitrification of the granulate takes place. For example the porosity of the granulate can be adjusted such that gas residues are still able to escape. Precompacted granulate can be vitrified slower and more carefully and it results in pore free quartz glass granulate having a theoretical density.
In a preferred method of proceeding the granulate is exposed to a reactive atmosphere in a first sintering step. This thermal and chemical treatment of the porous granulate allows, in addition to the precompacting, also a chemical alteration of the granulate. For example, thermal chlorinating or oxygen treatment of the granulate allows removal of contaminants such as metallic compounds, water, OH groups and/or carbon residue, or the granulate may be charged with dopants. Advantageously, granulate of an average particle size between 50 xcexcm and 300 xcexcm is used. The lower limit is determined by the pourability of the granulate, the upper limit by the increase in heat energy necessary for the vitrification of the granulate due to the increasing granulate particle size.
Particularly advantageous has been the use of this process for the vitrification of granulate produced by spray granulation by means of centrifugal dispersion. Such granulate, distinguished by a comparatively small average particle size can hardly be vitrified without agglomeration when stationary vitrification methods known in the art are used. Fine glassy granulate is especially suitable as filler in sealing compounds for electronics applications.
As the starting material for the process according to the invention the granulate produced by means of spray granulation advantageously has an average particle size ranging between 5 xcexcm and 150 xcexcm.
As far as concerns the specific surface of the granulate as used in the process according to the invention it has been shown to be advantageous when it is in the range between 5 m2/g and 100 m2/g, as measured by means of the BET (Brunauer-Emmett-Teller) method.
Depending on the intended application of the quartz glass granulate a method of proceeding is preferred where the vitrification of the porous granulate takes place under reducing conditions. In the simplest case, reducing conditions are created during the vitrification of the granulate in an oxyhydrogen flame by way of a hydrogen surplus. It has been shown that the OH content in the glassy quartz glass granulate can be reduced at least partially by tempering under higher temperatures and insofar the OH content of the quartz glass granulate can be adjusted to a certain extent.
In an equally preferred variant of the process the vitrification of the porous granulate takes place in oxidating conditions. Oxidating conditions are in the simplest case established during the vitrification of the granulate in an oxyhydrogen flame by means of an oxygen surplus. It has been shown that the OH content in the glassy quartz glass granulate is for the greatest part firmly bonded and can be removed during tempering at higher temperatures only to a small degree or not at all. This is primarily advantageous when constant characteristics of the quartz glass granulate are desired in the intended application.
The process according to the invention will be explained below in more detail by way of an example and a drawing.